const schemaKeywordTypes = {
  multipleOf: 'number',
  maximum: 'number',
  exclusiveMaximum: 'number',
  minimum: 'number',
  exclusiveMinimum: 'number',

  maxLength: 'string',
  minLength: 'string',
  pattern: 'string',

  items: 'array',
  maxItems: 'array',
  minItems: 'array',
  uniqueItems: 'array',
  additionalItems: 'array',

  maxProperties: 'object',
  minProperties: 'object',
  required: 'object',
  additionalProperties: 'object',
  properties: 'object',
  patternProperties: 'object',
  dependencies: 'object'
};

export function inferType(schema) {
  if (schema.type !== undefined) {
    return Array.isArray(schema.type) ? schema.type.length === 0 ? null : schema.type[0] : schema.type;
  }
  const keywords = Object.keys(schemaKeywordTypes);
  for (var i = 0; i < keywords.length; i++) {
    let keyword = keywords[i];
    let type = schemaKeywordTypes[keyword];
    if (schema[keyword] !== undefined) {
      return type;
    }
  }

  return null;
}